Electronic burner control apparatus



Aug. 11, 1953 M. R. SMITH ET AL 2,648,375

ELECTRONIC BURNER CONTROL APPARATUS Filed Dec. 27, 1949 HARVEY J. SMITH (Iitomeg F- "91- w L l i 93 96? g 76 94 95 I 80x 8| l 88% '33 I i Z 7* a2 l I3I I I21! I l 62 25 65 I XJ/r l 80 6| I 1408 I 45 I 47 1 Z 123 f L425 I ZSnventor MYRON- R SMITH Patented Aug. 11, 1953 ELECTRONIC BURNER CONTROL APPARATUS Myron R. Smith, Minneapolis, and Harvey J.

Smith, St. Louis Park, Minn., assignors to Minneapolis-Honeywell Regulator Company, Minneapolis, Minn., a corporation of Delaware Application December 27, 1949, Serial No. 135,122

Claims. 1

The present invention is concerned with an automatic controller for a fuel burner and more particularly with that type of control which utilizes electronic circuits in its controlling function.

In certain types of fuel burner installations, particularly those employing a fuel which is highly explosive, it is necessary to provide controls which will respond quickly to any condition which will be dangerous if allowed to continue. By providing electronic flame detection in control circuits, the speed of response to dangerous conditions may be maintained at a high value and may be readily adjusted to suit the particular installation. When such electronic circuits are utilized it is also necessary to insure that operation of the burner will be prevented if the control is not functioning properly. Further, the control circuits must be so arranged that during ofi periods of the control burner, the control circuits will not accumulate any charges which might adversely affect operation upon a subsequent demand for burner operation. Another item of importance in such a control as the present one is the simplicity of design and the obtaining of a low voltage switching circuit so that an ordinary control thermostat may be utilized for initiating operation of the control.

It is therefore an object of the present invention to provide a new and improved electronic burner control.

A further object of the present invention is to provide a new and improved burner control utilizing an impedance timing circuit for shutting down the control burner upon a flame failure.

Still another object of the present invention is to provide a new and improved electronic burner control having a low voltage switching control circuit.

A still further object of the present invention is to provide an improved burner control where the control apparatus will maintain the burner inoperative if there is a flame failure or a false indication of flame.

Still another object of the present invention is to provide a condition control apparatus having a pair of transformers, one of which is continuously energized from a source of power and the other of which is arranged to be energized by said one transformer upon a need for operation of a condition changing means.

These and other objects of the present invention will be obvious upon a consideration of the accompanying specification, drawings and appended claims.

2 In the single figure, it will be noted that there is provided a thermostatic device It) for controlling the fiow of fuel to a fuel burner l I. The thermostat device H1 acting with the control ciring 8 will initiate burner operation by energizing an electromagnetic pilot valve 12, an ignition transformer l3, and a main valve l4 after a pilot flame appears on the pilot nozzle i5 and impinges on a flame electrode I S. The complete operation of the apparatus in conjunction with the control fuel burner will be considered after considering the components included within the housing 8.

Supplying power to the control apparatus are a pair of input power lines 29 and 2| connected to a primary 23 of a transformer 22. The transformer 22 has a pair of secondary windings 24 and 25, the latter of which is tapped at 26. The secondary 24 has a relatively low voltage thereon and is used for supplying an energizing voltage to the filament heaters 3!, 32, 33 and 34. Each of these filament heaters is associated with a different one of the electron discharge devices of the present apparatus.

The secondary winding 25 is connected to the flame detection circuit of the present apparatus. Included in the flame detection portion of the apparatus are a pair of electron discharge devices 48 and 45 which may be separate sections of a dual triode tube. 'The discharge device 40 comprises an anode ii, a control electrode 42, and a cathode &3. The discharge device 45 comprises an anode A5, a control electrode 41, and a cathode 48. The cathodes i3 and 48 are adapted to be heated by a pair of the filamentary heaters 3| 34 energized by the secondary 24.

Located on the input to the discharge device 40 is a condenser 59 which is adapted to assume a charge dependent upon the presence or absence of a flame, in a manner that will be understood in the operation of the apparatus as explained. Connected between this condenser and the control electrode 42 is a resistor 52 which cooperates with a condenser E! to form a filter network on the input of the discharge device 40. A resistor 53, connected between the control electrode 42 and cathode 43, is a conventional grid leak resistor. Located on the output of the discharge device id and on the input of the discharge device 45 is a resistor 54. A further resistor 55 is used in controlling the control electrode potential of the control electrode 41. Located in the output of the discharge device 45 is a control relay 58 comprising a relay winding 59, a pair of switch blades :60 and 65, the

latter of which are normally biased out of engagement with contacts 62 and by means not shown. The switch blade ti is normally biased into engagement with a contact Connected in alternating current by-passing relation to the winding 55'] is a condenser 65 which serves to fllter out the alternating component of the current flowing in the output circuit of the discharge device 6-5.

The energization of the control burner is initiated by the electronic control circuits indicated generally by the numeral 75. The supplying of power to these circuits is by way of a transformer H having a primary winding l2 and a secondary winding 73. Included in these circuits are a pair of electron discharge devices l5 and which may also be two sections of a dual triode tube. The discharge device l5 comprises an anode ill, a control electrode it, and a cathode 75, the latter of which is adapted to be heated to be eleotron emissive by one of the filament heaters energized by the secondary 2d. The discharge device 15 comprises an anode 85, a control electrode 8!, and a cathode 32. Located in the output circuit of the discharge device 15 are a pair of series connected resistors 85 and 85. Connected between the junction of the two series resistors 85 and 86 is a further resistor 8i which may be adjustable and this resistor is also connected to the control electrode 8! of the discharge device lG. Connected between the control electrode 51! and. cathode 82 is a timing condenser 33. A shunting resistor 89 is adapted to be connected in parallel with the condenser 83 by a reset switch 9!]. Located in the output of the discharge device ?5 is a control relay 92 having a relay winding 95, and a pair of switch blades 95 and 95 which are normally biased, by means not shown, out of engagement with a pair of associated contacts 93 respectively. condenser 98 is connected in alternating current by passing relation to the winding 93 to by-pass the alternating component of the'current flowing in the output circuit of the discharge device it. A current limiting resistor 99 is provided in the energizing circuit of the subject apparatus.

Operation In considering the operation of the present apparatus, it will be flrstassumed that the apparatus is completely shut down with the thermo static device it not indicating the need for operation of the burner. Under this condition the solenoid valves l2 and M will be in a closed position. and the ignition device will deenergized. Further, the control relays 92 and 58 will be in the deenergized position. The relay 52 will be in the deenergized position inasmuch as there is no power applied to the transformer it when the room thermostat it is open.

The reason the relay 58 will be in the deenergized position will be understood upon a brief consideration of the operation of the flame detection portion of the apparatus. For a complete understanding of the operation of the apparatus, reference should be made to a copending application of Richard S. Fiegal, Serial No. 730,649 filed February 24, 1947, now Patent Number 2,556,961. Briefly, the present flame detector is so arranged that it utilizes the rectifying characteristics of a flame in the controlling of an electronic circuit. When there is no flame appearing at either the pilot nozzle B5 or the main burner l i, there will be no rectification in a flame circuit that may be traced from the tap 26 4 through condenser 55, conductor Bill, flame rod it, the gap between 65 and the burners H and i5 which are grounded at W2, ground Hi3, and conductor 504 back to the lower terminal of secondary 25. If there is no flame present across the gap described above, there will be no direct current charge built upon the condenser which is in the flame circuit traced. With no charge on the condenser 55, the control electrode 42 of the discharge device t9 will assume the same potential as the cathode 43 and therefore the discharge device will be conducting current on the alternating current half cycles of the power supply when the anode M is positive. The

. current flow may be traced from the tap 26 through conductor E65, resistor 54, conductor H55, anode 4i, cathode 43, and conductor I04 back. to the lower terminal of secondary 25. It will be noted that the last traced current flow was through the resistor 54 and with this current flowing the voltage drop across this resistor will be such that the upper terminal will be negative and the lower terminal will be positive. In-

. asmuch as the control electrode 4'! is connected to the upper terminal of resistor 54 and the oathode AS is connected to the lower terminal thereof, there will be a biasing voltage applied to the input of the discharge device 45 which will maintain the current flow in this discharge device below the value necessary to effect energization of the relay 58. Thus, with no flame present, the relay 5% will be deenergized.

If a flame should be present at the pilot nozzle l5 or at the main burner ii, there will be a current flow in the flame circuit that was traced above and the conventional current flow will be greater in the direction traced so that the condenser will assume a charge such that the upper terminal is positive and. its lower terminal is negative. With this charge on the condenser 5?}, a biasing voltage will be applied to the input of the discharge device ill so that this device will have its control electrode 52 negative with respect to the cathode 33 and the device will no longer be conducting sufliciently to maintain the discharge device 435 nonconductive. With the discharge device 15 conductive, the relay 58 will move to the energized position to indicate the presence of a flame at either the pilot nozzle or the main burner. The current flow circuit for the discharge device 55 may be traced from the upper terminal of the secondary 25 through conductor Hll, relay winding 59, conductor I08, anode t6, cathode 48, and conductor to the tap 25 of secondary 25. With sufficient current L flowing in the last traced circuit, the relay 58 should become energized. Inasmuch as the operation of the flame detector depends upon the rectifying characteristics of a flame, it will be obvious that the apparatus will not detect the presence of flame if the electrode it should become grounded on the burner it or on the pilot nozzle l5 or if the flame gap should become bridged by a non-rectifying impedance. This is r one of the added'features of safety of the present apparatus. Thus, only when there is a rectifying impedance across the flame gap will it be possible for the relay 58 to become energized.

As soon as the space wherein the thermostatic device it is located begins to cool, the thermostatic device it will close its switching circuit and will connect the primary winding 72 in energizing relation to the secondary winding 25 so that power may be supplied to the circuitlfl. This energizing circuit may be. traced from the lower terminal of secondary 24 through conductor IIll, thermostatic device I0, conductor II I, conductor H2, primary I2, and conductors H3 and H4 to the upper terminal of secondary 24. As pointed out above, the voltage on the secondary 24 is relatively low and is used for supplying the energizing voltages for the filaments 3I to 34. Inasmuch as this voltage is low it is possible to use an ordinary thermostat circuit of the low voltage type which doesnot require the special installation requirements normally present in thermostat circuits that operate at high voltages.

With the primary 12 energized, a voltage will appear on the secondary I3 and this voltage will be sufficient to cause energization of the control relay 92 as the discharge device 18 will be initially conducting. The conducting circuit may be traced from the upper terminal of secondary I3 through conductors II5 and H6, winding 93, anode 80, cathode 82, and conductor III to the lower terminal Ofsecondary 13. The discharge device I5 will be conducting because initially the control electrode 8I will be at the same potential as the cathode 82 because of a circuit that may be traced from the control electrode 8| through conductor II 8, and resistors 81 and 88 to the cathode 82. Current will also flow through the discharge device I5 in a circuit that may be traced from the lower terminal of secondary 13 through conductor I I1, resistors 86 and 85, anode I'I, cathode I9, and conductor -I I5 to the upper terminal of secondary I3. While initially there is current flowing in this last traced circuit upon the closure of the thermostatic device I8, the voltage drop from the resistors 85 and 86 will have no effect upon the potential of the control 8I because of the fact the condenser 88 is conneced between the control electrode 8| and cathode 82 and must assume a charge before the current flow in the discharge device 16 will be appreciably affected.

As soon as the relay 92 has become energized, the switch blades 94 and 95 will move into engagement with their associated contacts 96 and 97 respectively. When the blade 95 engages contact 91, an energizing circuit may be traced to the pilot solenoid valve I2 and ignition transformer I3 from the input power line 28 through conductor I20, switch blade 95, switch contact 97, conductor I2I, junction I22, conductor I23, solenoid valve I2, and conductor I24 to the other input power line 2I. The circuit to the ignition transformer is the same as that for the pilot valve I2 up to the junction I 22 where the circuit is traced through the switch blade 8 I switch contact 54, conductor I25, ignition transformer I3, and conductor I24 back to the other input power line 2 I. With the solenoid valve I2 energized and ignition transformer I3 energized, gas will flow to the pilot nozzle I5 and be ignited by the ignition transformer I3. The resultant pilot flame will bridge the gap between the nozzle I5 and the flame electrode I6 so that flame will be detected by the flame detector, in the manner explained above.

As soon as flame is detected at the pilot burner, the relay 58 will become energized and the switch blades 50 and 6| will move into engagement with their associated contacts 62 and 63 respectively. When the blade 6| engages contact 63 an energizing circuit is completed to the main solenoid I4 and this circuit may be traced from the input power line 20 through conductor I20, switch blade 95, switch contact 91, conductor I2I, junction I22, blade 6I, contact 63, conductor I26, solenoid valve I4, and conductor I24 back to the other input power line 2I. With the solenoid valve I4 energized, gas will be supplied to the main burner II. When the blade BI moves out of engagement with contact 84 and into engagement with 63, the circuit to the ignition transformer is broken so that now only the pilot solenoid valve I2 and the main solenoid valve I4 are energized.

When the relay 58 becomes energized, and the blade 68 moves into engagement with contact 82, the control electrode 8| of the discharge device IB is connected to the cathode 82 in a circuit that may be traced from the control electrode SI through conductor II8, conductor I30, switch blade 88, switch contact 62, conductors I3I, and I32, contact 96, switch blade 94, ground I33, ground I34, conductor I I0, thermostatic de-- vice III, and conductors I II and II! to the oath-- ode 82. With this connection, the condenser 88' is shorted out and it is insured that the discharge device IE will remain in the conducting condition so that the relay 92 will remain energized.

The apparatus will remain in the energized condition until such time as the thermostatic: device I0 no longer indicates a need for burner operation. When there is no longer a need for burner operation, the thermostatic device ID will open its circuit and the energizing circuit to the primary I2 of transformer II will be opened so that the power applied to the electronic control circuit III will be removed and the relay 92 will become deenergized, thus removing any source of charging voltage for condenser 88. When the relay becomes deenergized the switch blade 85 will move out of engagement with contact 91 and will open the energizing circuit to the pilot solenoid valve I2 and the main solenoid valve I4 so that the pilot burner and the main burner will be shut down. As soon as the flame disappears from the pilot burner and the main burner, there will no longer be a flame impinging upon the flame electrode I6 and the flame detector will no longer indicate the presence of flame so that the relay 58 Will also move to the deenergized position to again assume the position shown upon the drawing.

Operation upon a failure of combustion In the operation considered thus far, it has been assumed that the entire apparatus has been functioning properly and flame has been present when operation of the burner has been desired. Considering now the functioning of the apparatus in the event there should be a failure to initiate combustion or a failure of combustion after it has once been established, it is desired that the apparatus be rendered inoperative within a predetermined time after the apparatus indicates a combustion failure. Upon the energization of the electronic control circuit 19 by the closing of the thermostatic device I 8, and energization of control relay 92, there should be a flame appearing at the pilot nozzle I5 upon the energization of the pilot solenoid I2 and the ignition transformer I3 by the relay 92. If no flame appears on the pilot nozzle I5 the relay 58 will remain in the deenergized position. It will thus be noted that the timing condenser 88 will begin to assume a charge which is directly proportional to the voltage existing across the resistor 86 in the output circuit of the discharge device I5. The time that it will take for this condenser 88 to assume the full voltage on the resistor 86 will be dependent upon the size of the condenser 88 and that of the resistor 81 which is connected in series with the charging circuit of the condenser 88. If a flame is not detected within the predetermined time, determined by the time constant of the resistor 81 and condenser 88, the control electrode 8! will be biased negative with respect to the cathode 82 and the current flow in the discharge device 16 will drop below the point necessary to maintain the control relay 92 energized. When the relay 92 becomes deenergized the pilot burner and the ignition device will be deenergized and will remain deenergized until such time as the condenser 88 is discharged. In order to discharge the condenser 88, it is but necessary to close the reset switch 90 which connects the resistor 89 directly in parallel with the condenser 88. This will discharge the condenser so that it will be again possible to reenergize the relay 92 upon the current flow in the discharge device '16 rising to a predetermined level. As soon as a flame is established at the pilot burner, the condenser 88 is shorted out by the engaging of the switch blade 68 to the contact 62 which is in circuit with the blade 94 and the contact 96 in the circuit that was traced above in considering the normal operation of the apparatus.

If the apparatus has been energizedand has been operating normally for a period and there is then a flame failure, it will be noted that the flame detection relay 58 will move to the deenergized position and the shunting circuit for the condenser 88 will be removed. When the relay 58 moves to the deenergized position, the deenergized circuit through the ignition transformer I3- is again completed to the switch blade 6t andcontact 64. If a flame is not reinitiated by the energization' of the ignition transformer [3, the condenser 88 will assume a charge in the aforementioned predetermined time and will bias the discharge device 16 below the current conducting point necessary to maintain the relay 92 energized. With the relay 92 deenergized, the entire apparatus will be shut down until such time as the reset switch 90 is depressed and the condenser 88 is discharged so that it is again possible for current to flow through the discharge device 18 to energize relay 92. Again, if upon the reenergization of the relay 92 there is a flame :appearing at the pilot nozzle !5, flame will be .detected and the relay 58 will become energized 130 again shunt out the condenser 88 to maintain the discharge device 19 in a conducting condition.

Operation upon false indication of flame If for any reason the fiame detection relay should remain in the energized position to thusly indicate the presence of a burner flame when in actuality the burner was not energized, it is desired that the apparatus be maintained inoperative to prevent energization of the relay 92 and the energization of either of the solenoid valves i2 or I4. If the relay 58 is in the energized position falsely indicating flame, the switch blade 60 will be engaging the contact 62. If the thermostatic device It subsequently closes, there will be applied to the discharge device 75 an alternating current bias voltage which is phased so that the control electrode 8| will be negative when the anode 80 has a positive half cycle applied thereto. Ihis biasing circuit will be cornpleted only upon the closing of the thermostatic ,device and may be traced from the control electrode 8| through conductor H8, conductor I30, switch blade 60, switch contact 62, conductor l3l, resistor 99, conductor H3, to the upper terminal of primary 72. The lower terminal of primary I2 is directly connected by means of conductors H2 and H! to cathode 82. When the alternating current on the upper terminal of the primary I2 is negatively phased, the upper terminal of the secondary 13, which is connected to the anode of the discharge device I6, will be positively phased. It will thus be seen that it will be impossible for the discharge device 16 to become conductive upon the closing of the thermostatic device ii! as long as the switch blade 60 is engaging contact 62. As soon as the fault causing false operation of the relay 58 has been removed, it is possible to initiate operation of the burner in the normal manner.

Conclusion In conclusion, it will be seen that there has been provided in the present invention a simple and reliable burner control that may be readily controlled by a low voltage switching circuit. Also provided is an improved type of quick acting arrangement for shutting down the apparatus in the event there is any flame failure or in the event there is a false indication of flame by the flame detection portion of the apparatus. While we have shown one particular embodiment of the invention, it is to be understood that it has been for illustrative purposes only and since many modifications can be made by one skilled in the art, we intend our invention to be limited solely by the scope of the appended claims.

We claim as our invention:

1. Apparatus for controlling a fuel burner,-

comprising in combination, a source of power comprising a transformer having a high and a low voltage secondary winding, an electronic flame detector connected to be energized by said high voltage secondary winding and comprising a plurality of discharge devices having input and output circuits and having filament heaters, the latter of which are connected to said low voltage secondary winding, an electronic control circuit for initiating operation of the fuel burner, a sectransformer comprising a primary and secondary winding, means connecting said last named secondary winding to energize said control circuit, and circuit means adapted to be completed by low voltage switching means indicative of the need for burner operation for connecting said primary Winding to be energized by said low voltage secondary winding.

2. Apparatus for controlling a fuel burner, comprising in combination, an electronic flame detector comprising an electronic control device, a source of power comprising a transformer having a primary winding, a first secondary winding whose voltage is greater than a voltage applied to said primary, and a second secondary winding whose voltage is less than the voltage applied to said primary, means connecting said first secondary winding to said control device, an electronic control circuit means for initiating burner operation, low voltage switching means indicative of the need for burner operation, and circuit means adapted to be completed by said switching means for connecting said control circuit means to said second secondary winding.

3. Apparatus for controlling a fuel burner, comprising in combination, an electronic flame detector comprising an electron discharge device having an anode-cathode circuit, a source of power comprising a transformer having a primary winding, a first secondary winding whose voltage is greater than a voltage applied to said primary, and a second secondary winding whose voltage is less than the voltage applied to said primary, means connecting said first secondary winding to the anode-cathode circuit of said discharge device, an electronic control circuit means for initiating burner operation, a step-up transformer for supplying energy to said control circuit means, and circuit means adapted to be completed by a low voltage switching means indicative of the need for burner operation for connecting said step-up transformer to be energized by said second secondary winding so that the only voltage on said circuit means will be from said secondary winding.

4. Apparatus for controlling a fuel burner, comprising in combination, an electronic discharge device having an anode, cathode, control electrode, and cathoce heater, a power transformer connected in energizing relation to said anode and cathode, a rectifying connected to said transformer, an electrical time delay network having a chargeable element connected to control electrode and to rectifying means, said rectifying means when energized normally tending to charge said chargeable element to a polarity to effect nonconduction of said discharge device, a source of power, means permanently connecting said source to said cathode heater, relay means in circuit with said anode for initiating operation of the burner when said discharge device is conductive, circuit means adapted to be completed by means indicative of the need for burner operation for connecting said power transformer to said source, said discharge device when said circuit means is completed energizing said relay means for a predetermined length of time dependent upon the charging time of chargeable element, burner operation responsive means rendering said chargeable element ineffective in said time delay network when said burner is operating properly, and means including said circuit means when completed for ap plying a biasing potential to said discharge device to render said device inoperative when said responsive means is indicating burner operation falsely.

5. Apparatus for controlling a fuel burner, comprising, a source of power comprising a transformer having a pair of secondary windings, an electronic flame detector connected to be energized by one of said secondary windings and comprising a first electron discharge device having a filament heater, the latter of which is connected to the other of said secondary windings, an electronic control circuit means for initiating operation of the fuel burner including a second electron discharge device having a control electrode, circuit means adapted to be completed by means indicative of the need for burner operation for connecting said control circuit to be energized by the second of said secondary windings, and means including said flame detector for connecting said other secondary to the control electrode of said second electron discharge device to prevent the initiation of operation of the fuel burner when said flame detector is falsely indicating proper burner operation.

6. Apparatus for controlling a fuel burner, comprising in combination, a source of power comprising a transformer having a first secondary winding and a second low voltage secondary winding, an electronic fiame detector connected to be energized by the first of said secondary windings, an electronic circuit means for initiating operation of the fuel burner, a second transformer for supplying energy to said last named circuit means, and further circuit means adapted to be completed by a low voltage switching means indicative of the need for burner operation for connecting said second transformer to the second of said secondary windings.

7. Apparatus for controlling a fuel burner, comprising in combination, a first electronic circuit means including an electron discharge device having a control electrode, first relay means connected to and controlled by said discharge device, a source of power, control circuit means adapted to be completed by means indicative of the need for operation of the burner, said circuit means connecting said first electronic circuit means to said source when said circuit is completed by said indicative means, a second electronic circuit means for indicating proper operation of the burner, second relay means connected to and controlled by said second circuit means, means including said second relay means connected to the control electrode of said discharge device for maintaining said discharge device inoperative when said second relay is in operating position and said indicative means is subsequently actuated, a source of direct current controlled by said indicative means, and circuit means including time delay means connecting said direct current source to the control electrode of said discharge device for rendering said discharge device ineffective after a predetermined time delay when said second electrical circuit means does not indicate proper burner operation, said source of direct current being rendered inoperative when said control circuit means is not completed bysaid indicative means.

8. Apparatus for controlling a fuel burner, comprising in combination, a source of power comprising a transformer having a first secondary winding and a second low voltage secondary winding, an electronic fiame detector" connected to be energized by said first secondary winding and comprising a plurality of discharge devices each having an input and output circuit, filament heaters for said discharge devices Which are connected to said second secondary winding, an electronic control circuit means for initiating operation of the fuel burner, and circuit means adapted to be completed by means indicative of the need for burner operation for connecting said control circuit means to be energized by said second low voltage secondary winding.

9. Apparatus for controlling a fuel burner, comprising in combination, a source of power comprising a transformer having a first secondary winding and a second low voltage secondary winding, an electronic flame detector connected to be energized by the first of said secondary windings and comprising a plurality of discharge devices having input and output circuits and having filament heaters, the latter of which are connected to the second of said secondary windings, an electronic control circuit means for initiating operation of the fuel burner, a second transformer comprising a primary and secondary winding, means connecting said last named secondary Winding to energize said control circuit means, and circuit means adapted to be completed by means indicative of the need for burner operation for connecting said primary winding to be energized by said second low voltage secondary winding.

10. Condition control apparatus, comprising in combination, first means for initiating operation of condition changing means, said means comprising an electron discharge device having a heater therefor, a source of power, means continuously connecting only said heater to said source of power, first circuit means adapted to be completed by means indicative of the need for operation of the condition changing means, a power transformer, means including said first circuit means connecting said discharge device to said source through said power transformer, a source of varying biasing voltage from said transformer, second circuit means connecting said source of varying biasing voltage in controlling relation to said discharge device and adapted to be effective to render said discharge device inoperative only upon the completion of said first circuit means, relay means responsive to the proper operation of said condition changing means for altering said second circuit means to thereby render said source of varying biasing voltage ineffective to control said discharge device upon said condition changing means operatin properly, means independent of said transformer for energizing said relay means under control of said condition changing means, and means including said first circuit means when completed for applying a second biasing voltage to said discharge device to render said device inoperative when said relay means is falsely indicating proper operation of said condition changing means.

MYRON B. SMITH.

HARVEY J. SMITH.

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